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Today, we can use SCRAM authentication for Kafka Brokers when the cluster uses ZooKeeper (ZK) for the quorum servers. This is possible to bootstrap by first setting up the ZK servers and setting the inter-broker communication password by directly updating them on the ZK before the Kafka Brokers are started. See Configuring SCRAM for details. We wish to implement some similar mechanism for storing the Kafka Broker authentication credentials for SCRAM when the cluster uses KRaft for the quorum servers. We want these credentials to be stored before the Kafka cluster starts for the first time.

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I propose the SCRAM_CREDENTIAL argument will be a key value pair where the key is one of the SCRAM mechanisms supported, either SCRAM-SHA-256 or SCRAM-SHA-512, and the value is a set of key value pairs of parameters to populate the UserScramCredentialsRecord. The SCRAM_CREDENTIAL argument is very similar to the argument passed to the kafka-config tool for configuring SCRAM in a ZK cluster. See Configuring SCRAM for details. 

The subarguments for the SCRAM_CREDENTIAL require a "user" key and either a "password" key or a "saltedpassword" key. If using the "saltedpassword" key you must also supply an "iteration" key and a "salt" key. The "iteration" and "salt" key are otherwise optional. However if they are not supplied, "iteration" count will default to 4096 and the "salt" will be randomly generated. The value for "salt" and "saltedpassword" is base64 encoding of binary data.

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